In recent times, there has been an increasing interest in the use of coherent optical communications for high-capacity and long-haul transmission, particularly at bit rates of 40 Gb/s and higher.
Coherent optical communication systems provide a number of potential advantages over more-conventionally deployed incoherent (i.e. intensity-modulated direct-detection) systems. These include improved receiver sensitivity, the ability to employ more sophisticated and spectrally efficient modulation formats, and the ability to compensate for linear transmission impairments, such as Chromatic Dispersion (CD) and Polarisation Mode Dispersion (PMD), within the electronic domain, for example using digital signal processing within the optical receiver.
As a result, the ultimate limitation on channel capacity within a coherent optical communications link is generally the nonlinear transmission impairments. While silica optical fibres exhibit very low nonlinearity, even this small effect becomes significant at high transmission powers, and over long transmission distances. Degradation of transmitted signals due to fibre nonlinearity increases with transmission power, and accordingly the channel capacity of an optical link cannot be increased indefinitely merely by increasing the signal launch power. Fibre nonlinearity is a deterministic phenomenon, which translates intensity fluctuations in the propagating signals into phase errors via the Kerr effect. However, compensating for fibre nonlinearity is challenging in the presence of CD, because the intensity waveform evolves along each fibre span.
In Wavelength Division Multiplexing (WDM) systems, inter-channel nonlinear impairments, predominantly Cross-Phase Modulation (XPM), provide the greatest compensation challenge. The inter-channel impairments are generated by intensity fluctuations in neighbouring WDM channels, and may vary not only due to CD, but also because WDM channels may be added, dropped and/or switched within a network, resulting in corresponding dynamic changes to the nonlinear interactions.
Many deployed optical fibre transmission systems comprise in-line dispersion compensation or management elements, such as spans of Dispersion Compensating Fibre (DCF). These systems are designed to minimise the accumulation of CD, particularly for the benefit of incoherent optical channels for which the compensation of chromatic dispersion in the electrical domain is not feasible. However, dispersion-managed systems can create their own problems when used to carry coherent optical channels, due to the impact of dispersion management on the nonlinear impairments. For example, coherent modulation formats, such as Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) may exhibit very high peak-to-average power ratio, which results in strong intensity fluctuations throughout a transmission link. In systems employing in-line dispersion compensation, these fluctuations have the same waveform within each span of the link. As a result, phase errors add coherently from span-to-span, leading to strong nonlinear distortion. This distortion results not only from XPM in WDM systems, but also from Self-Phase Modulation (SPM) within each individual channel.
Methods and apparatus for mitigating the effects of SPM in CO-OFDM transmission links are disclosed in U.S. patent application Ser. No. 12/445,386, to Lowery et al, having a filing date of 20 Dec. 2007. However, these methods are based on knowledge of the signal waveform in an individual transmitted channel, and are therefore less effective in the presence of XPM due to adjacent channels in WDM systems, where the adjacent channel waveforms are generally unknown.
There is, therefore, an ongoing need for further improvements in nonlinear compensation in coherent optical communications systems. Desirably, compensation techniques should be implementable in the electrical domain, for example using digital signal processing methods. It is also desirable that new nonlinear compensation methods are applicable to systems both with and without in-line dispersion compensation and/or management.
It is, accordingly, an object of the present invention to meet the abovementioned needs.